High-temperature rechargeable batteries, such as sodium-based thermal batteries like sodium metal halide or sodium sulfur cells, have a number of components that need to be sealed for the cell to work. Sodium metal halide batteries, for instance, include a sodium metal anode and a metal halide (NiCl2 for example) cathode. Beta″-alumina solid electrolyte (BASE) separator is used to separate the anode and cathode. The solid electrolyte allows the transport of sodium ions between anode and cathode. A secondary electrolyte (NaAlCl4) is also used in the cathode mixture. The cathode mixture consists of nickel and sodium chloride along with other additives. The cathode mixture is contained inside the BASE tube, which is closed on one end.
The present design of Na—NiCl2 battery cell entails having the open end of this beta-alumina ceramic tube joined to an alpha-alumina collar using a glass seal. Spinel, zirconia, yttria, or other ceramic insulators, or combinations thereof, may also be used as a collar material in Na—NiCl2 batteries. The collar is in turn joined with nickel rings with the help of thermal compression bonding (TCB). TCB is achieved through metallizing the collar. The design of the present cell demands this seal to be resistant towards molten sodium and molten halide (sodium melts at 98° C. and NaAlCl4 melts at 157° C.). The glass seal and TCB are two of the weak links in the present design for a path to long life: the glass seal and TCB encounter corrosion from sodium and halide and, because of this, are found to degrade over time.
There are two ways to address this problem; one is by improving the glass seal and TCB in terms of degradation from sodium and halide corrosion; and second is by completely eliminating the glass seal and TCB in the design of the cell. The use of this glass seal can be eliminated by using a graded ceramic (beta-alumina tube with alpha-alumina header) tube. However, in the design where this graded tube is used, the nickel ring cannot be joined with the alpha-alumina collar using a TCB-like process. Therefore, alternate joining technologies are necessary.
Active brazing is a procedure in which one of the components from a braze alloy reacts with ceramic and forms an interfacial bond. The requirement of a braze alloy for use in high temperature rechargeable batteries is high corrosion resistance towards sodium and halide. Conventionally, brazing is done through metallization in combination with a braze alloy. However, metallization (for example with Mo) is typically carried out at a temperature ˜1550° C., a temperature too high for beta-alumina, as it starts losing soda (Na2O). Therefore, metallization is not an appropriate procedure for the beta-alumina tube found in these cells. Further, the metallization/TCB process is complicated and expensive. Active brazing has been known in the literature to join ceramic to metal, but there are not many active braze alloys (ABAs), particularly high temperature (900-1200° C.) ABAs and resistant to corrosion from sodium and halide available commercially.
There continues to be a growing need in the art for high performance metal halide batteries with lower fabrication costs. Prior attempts for achieving this have utilized reticulated carbon foams and meshes. However, these materials frequently do not allow for even distribution across the cathode. Additionally, they are often more expensive than the nickel they are trying to replace. The methods of introducing these materials to the cathode can be quite arduous and difficult to put into commercial large scale operation. Thus, it may be desirable to have an electrode material that maintains the performance of the battery, but allows for a reduction in costs over those materials currently available.